The challenge facing non-cryogenic gas separation is to have adsorbents with higher capacity and selectivity. It is well established that selective gas adsorption occurs at cation ion sites within crystalline microporous solids (ie. zeolites), where a more strongly adsorbed component of a gas mixture will physically adsorb, leaving a gas stream enriched in the less strongly adsorbed component. An example of this is the equilibrium controlled adsorption of N.sub.2 from air to give a stream enriched in O.sub.2. It is known that zeolites are useful for this type of separation, and it is well known that zeolites with high charge density cations, eg. Ca or Li are effective for this separation. The factor limiting the use of zeolites for this application is that in zeolites (crystalline aluminosilicates) a limiting amount of cations is reached when these zeolites become Al saturated. This is defined at a Si/Al ratio of 1.0. If there were ways of modifying the framework array of aluminum to add more cations in accessible areas, these materials could have greater utility for gas separations.
Enhancements in N.sub.2 adsorption and N.sub.2 /O.sub.2 selectivity for gas separations applications using Li X-zeolite adsorbents are well known. Chao in U.S. Pat. No. 4,859,217 describes this enhancement when the SiO.sub.2 /Al.sub.2 O.sub.3 ratio of an X-type zeolite is between 2.0 and 3.0 (Si/Al ratios of 1.0 to 1.5) and when 88 percent or more of the Li cations are associated with AlO.sub.2 tetrahedral units.
One skilled in the art would recognize that the X-zeolite adsorbents of Chao have the faujasite structure and would have a solid state .sup.29 Si magic angle spinning NMR spectrum consistent with that shown in Engelhardt and Michel, "High-Resolution Solid-State NMR of Silicates and Zeolites, John Wiley & Sons, 1987, pp 222-241.
The aluminum rich end member of this series, commonly known as low silica X (LSX), with a SiO.sub.2 /Al.sub.2 O.sub.3 ratio of 2.0, is well known to have a fixed and alternating arrangement of Si and Al tetrahedral framework atoms, leading to a uniform distribution of Al and Si throughout the crystal. This composition also has the maximum number of Li cations per unit cell and has the best adsorption properties found so far for a gas separation application, such as O.sub.2 VSA.
Mullhaupt in U.S. Pat. No. 5,441,557, and its divisional U.S. Pat. No. 5,554,208, show that when the cation and Al framework atoms are arranged in a symmetric (uniform) fashion throughout the entire crystal of zeolite the adsorption properties are maximized. According to Mullhaupt, semi-symmetric or asymmetric (non-uniform) Al or cation distributions throughout a zeolite crystal lead to inferior adsorption properties in this class of materials. According to this prior art, arrangements of Al atoms in a faujasite structure are a result of the Si/Al ratio of the particular composition. This notion of a fixed arrangement, would dictate both cation content and placement within a unit cell of the crystal.
However, It is possible to have solid state isomers of materials with the same bulk composition. These are defined as the aristotype and hettotypes according to Megaw (Helen D. Megaw Crystal Structures: A Working Approach, W. B. Saunders Company, Philadelphia, London, Toronto, 1973, pp. 282-285). The aristotype of faujasite is the silica polymorph, the all silica material, defined as ZDDAY by Hriljac et. al. (Hriljac, J. A.; Eddy, M. M.; Cheetham, A. K.; Donohue, J. A.; Ray, G. J. Journal of Solid State Chemistry 1993, 106, pp. 66-72). All compositions with the faujasite structure that contain both Si and Al in the framework are classified as hettotypes of this aristotype. Hettotypes are defined as having lower symmetry which are a result of various modifications of the aristotype, such as having different elements (or arrangements of elements) with a similar unit cell. Since Si and Al have very similar X-ray scattering characteristics, the X-ray diffraction (XRD) patterns of these various hettotypes are similar.
The X-type zeolites of this invention are hettotypes of the same framework structure found by Milton over 30 years ago. Milton's U.S. Pat. No. 2,882,244 defines zeolite X based on composition and diffraction. The X-type zeolites of the present invention are within the bulk compositional range (SiO.sub.2 /Al.sub.2 O.sub.3 =2-3) of Milton and have similar XRD patterns. For a given hettotype, with a fixed composition, numerous variations of the arrangement of Si and Al are possible throughout the crystal and will not significantly alter the XRD pattern to any great extent.
One analytical technique to measure these differences is .sup.29 Si NMR. Based on the integrated intensities of the Si(OAl) and Si(1Al) resonances in the solid state magic angle spinning (MAS) .sup.29 Si NMR pattern, we can divide the hettotypes for a given composition of X-type zeolites into two subsets. Type 1 hettotypes of faujasite (referred to hereafter as Type 1) are those in which the integrated intensity of the Si(1Al) resonance is greater than the integrated intensity of the Si(OAl) resonance, whereas Type 2 hettotypes (referred to hereafter as Type 2) of faujasite are those in which the integrated intensity of the Si(1Al) resonance is less than the integrated intensity of the Si(OAl) resonance. These differences are a result of differing arrangements of Al and Si throughout the crystal, at a fixed Si/Al ratio. Using the integrated intensities of the NMR spectrum, we can show that X-type zeolites which are defined as Type 2 hettotypes of faujasite have more Si framework atoms with 4 Si next nearest neighbors than Si framework atoms with 3 Si next nearest neighbors and 1 Al next nearest neighbor. This is a consequence of the NMR spectrum described above.
We classify the X-type zeolites of Milton and subsequently Chao, since Chao describes his materials as being prepared by the methods of Milton, with Si/Al ratios between 1.0 and 1.25 as being Type 1 hettotypes of faujasite. This means that these materials at a given composition in this range have a particular type of Al distribution, where there are typically not Al-rich regions within the crystal, but rather a homogeneous distribution of Al throughout. Surveys of the NMR literature are abundant with this hettotype of faujasite. These will be discussed shortly.
In Engelhardt (Z. Anorg. Alig. Chem. 1981, 482, pp 49-64), Table 1 on page 50 shows the peak intensities for an X-type zeolite having a Si/Al ratio of 1.18. In this case the peak intensity (height) for the Si(OAl) peak is higher than the Si(1Al) peak; however the integrated intensities give the relative number of Si atoms in a particular environment. This is discussed later (page 56, in the paragraph starting with Si/Al=1.18). He describes the integrated intensities as being 63.5: 23.1: 5.8: 3.8: 3.8. This would give equal numbers of Si(OAl) and Si(1Al) which is outside the definition of Type 2 hettotypes of faujasite, since it gives an equal number of Si(OAl) and Si(1Al) species in the crystals. This further adds support for multiple ways of arranging the Si and Al on the faujasite topology.
Klinowski in Nature (1981, 292, 228-230.) clearly states that his 1.19 material has only two peaks, the Si(4Al) and the Si(3Al) with amounts of 18.6 and 7.4 respectively (based on 24 T atoms per sodalite cage). These amounts of Si(4Al) and Si(3Al) are inconsistent with a Si/Al ratio of 1.19, given these amounts the Si/Al ratio would be 1.07. Discussion in the text indicates that they were unable to observe an Si(OAl) peak, but expect it to have intensity of 2.
Klinowski, J. Chem Soc. Faraday 2,1982, 78, pp 1025-1050, lists normalized peak intensities. However, since his bulk analysis value and the NMR value do not match, this can not be considered a valid representation of a Type 2 hettotype. He further goes on to model these data suggesting that either there should be no Si(2 and 1Al), or later in the discussion that there should be no Si(OAl). Considering all this taken together it is unreasonable to call this an example of a Type 2 hettotype.
Neither Milton nor Chao show NMR data; however, if one of reasonable skill uses the preparative methods described by Milton (as Chao stated was the route to his adsorbents) one makes Type 1 hettotype faujasite with the previously mentioned characteristic NMR patterns. Examples are cited below to illustrate this, and correlate the gas adsorption properties of subsequently ion exchanged materials.
Contrary to the prior art, the present invention has demonstrated that X-zeolites having silica rich regions within the crystal, provide an unexpected enhancement in gas separations, as will be set forth in greater detail below.